Electron-emitting electrode for electric discharge tubes



Patented Nov. 22, 1949 ELECTRON-EMITTING ELECTRODE'FOR ELECTRIC DISCHARGE TUBES Gregorius alohan- Lambertand'A'driaan A. "Padmos, .Eindhoven, Netherlands, assignors to General ElectricCompany, Schenectady,-N. Y.

No 'Drawing. Application April 27, 1946, Serial No. 665,636. In the NetherlandsMarch 21, 1942 Section 1, PubliciLavv 690, August8, 1946 Patent expires March-21, 1962 '3 Claims. 1

The invention relates to electron-emitting electrodes for electric discharge tubes and more particularly for discharge tubes filled with an ionizable medium such as gas or vapour. The invention furthermore relates to a method of manufacturing such electrodes.

Electron-emitting electrodes for electric discharge tubes are usually constituted by a metallic support, for example of tungsten, nickel or similar material, to which is applied a layer which readily emits electrons and which substantially consists of oxides. The support may be heated in this case either directly or indirectly. The emission'from these electrodes takes place owing to' the fact that during the'forming or seasoning a small amount of metal is formed from the oxides present at the surface.

Besides, it is known that a certainamount of metal may be added to these layers of oxide.

There has previously also been described an electron-emitting electrode for a discharge tube filled with gas'or vapour, which electrode is constituted by a support on which are present one or more oxides mixed with a larger quantity by weight of a highly refractory metal, for example tungsten.

In addition to the above-mentioned type, there are known electron-emitting electrodes with which, as-t'ne case may be with the'addition of a small amount of metal, oxides arepressed so as to form independent bodiesandsintered and are then surrounded on the outside by a conductor,

the aggregate thus obtainedacting as a cathode.

Such electrodes are utilized in discharge tubes filled with gas or vapour. It had also'beenproposed to utilize, not only in the last-mentioned tubes but also in high-vacuum tubes, mixtures of conducting substances and emitting non-conducting oxides, wherein the quantity of oxide was small relatively to the conducting substance'present in the material. These mixtures are also pressed and sintered and then utilized as a cathode either as'such' or only after having been drawn out to a smaller diameter. One known example of such a cathode is the so-called tungstenthorium cathode, built up from amixture of tungsten and thoria, in which-event the emission takes place owing to the fact that during the forming an extremelysmall quantity of thorium is 'formed.

These electron-emitting electrodes containing mixtures of oxides and metals, and electrodes wherein these mixtures are provided on .a substratum as well as electrodes wherein the sintered mixture as such acts as a cathode, entail drawbacks which chiefly consist in that the slight.

domain of application is limited thereby in some wayor other. In fact, if use is made of mixtures wherein, relatively to the conductive constituent, for example tungsten or nickel, little emitting oxide is present, the emission is comparatively If more emitting oxide is added it is no longer possible to manufacture drawn cathodes whilst when a proportionally large amount (if alkaline or alkaline earth metal oxide is present in the material, cathodes which have not been drawn are also unfit for use, more particularly in discharge tubes filled with gas or vapour, since this oxide disappears too rapidly from the material and considerably shortens the length of life of these tubes or reduces the possibility of application. If an oxide which disappears less rapidly,

such for example as thoria, would be taken 'for' this purpose, there arises the drawback that the electrode acquires an excessively low conductivity, due to which the material can no longer be used as an independent cathode but must be provided, either on the inside or on the outside, with an additional conductive body.

These drawbacks may be obviated by utilizing an electron-emitting electrode according to the present invention, which electrode contains a mixture of one or more metals of which at least one has a melting point which exceeds 1550 C. such, for example, as tungsten, molybdenum'or tantalum, and one or more readily emitting oxides such as the oxides of the alkaline oralkaline earth metals, wherein the quantity by weight of emitting oxide is small relative to the metallic components and wherein at least one of the metallic components has a work function which is smaller than that of molybdenum and ispresent in the mixture in a proportion of at least one per cent by weight.

Electron-emitting electrodes according to the invention ofler a large number of possibilities. For example, the same mixture may be utilized as a layer on a conductive core either for a directly or an indirectly heated cathode whilst from the same complex of substances can be manufactured by pressing and sintering a body which, as such and without the use of conductors surrounding this body, can act as'a cathode, a sufiicient length of life and a very satisfactory emission being always ensured in this case. The

existence of all these possibilities is due to the' fact that, on the one hand, comparatively little oxide is present, in which case also with the use of alkaline or alkaline earth oxide a slight evaporation and therefore 'a slight decrease of the emission take place, and that, on the other'hand,

the conductivity remains satisfactory and also :a satisfactory emission is obtained since one of the metallic components is at the same time a satisfactorily emitting substance and therefore not only facilitates the emission but also keeps the conductivity at a sufiicient value and reduces the possibility of evaporation to a minimum. The latter is achieved inter alia owing to the fact that the metal contains a sufficient proportion of metal with a melting point which exceeds 1550 C.; for this purpose use is preferably made of metals such as tungsten, molybdenum or tantalum. For the satisfactorily emitting metallic components use is preferably made of a metal having a high melting point, for example thorium; other metals, for example hafnium, tantalum or iron may also enter into account for this purpose.

This component is present in the mixture in a proportion of at least one per cent by weight. This percentage is preferably taken higher than the minimum quantity, for example or more than 20%.

Since a cathode according to the invention is very suitable for being utilized in the form of a pressed and sintered cathode, it is of particular importance for discharge tubes filled with gas or vapour such, for example, as high-pressure mercury-vapour discharge tubes. These electronemitting electrodes need only be placed on or in a holder and as such then act as cathodes. It has been found that very satisfactory results are obtained if a cathode thus formed consists of a mixture'of a metal with a very high meltingpoint, for example tungsten or molybdenum, one or more oxides of barium, strontium and calcium, and thorium as the conducting and satisfactorily emitting component, a very useful composition being a mixture of 65% by weight of tungsten, 25% by weight of thorium and 10% by weight of alkaline-earth oxide. Other compositions are also possible; for example, as the emitting substance there may be present thoria whilst the tungsten or the molybdenum may be replaced, either wholly or partly, by tantalum, nickel or similar metals. In this case the electrode may be composed as follows: 10% of thoria, 10% of tantalum, 10% of barium and calcium oxide, of hafnium and 5% of zirconium.

The method of manufacturing an electrode according to the invention is slightly different according as the mixture of substances as such, i. e. in the pressed and sintered and, as the case may be, drawn condition, has to act as an electronemitting electrode or is utilized as a layer on a conductive support.

In both cases the starting materials are intermixed in the form of powder; then this pow der is mixed with a binding agent and the paste thus obtained is either pressed into a determined shape and sintered or applied as such to a conductive core. In the first case the mixture of substances acts as a cathode, in the second case the cathode is formed by the core body whilst the above-described mixture of substances constitutes the emitting layer.

The invention will be explained more fully with reference to two practical examples wherein methods of manufacturing an electron-emitting electrode according to the invention are described.

Example I The starting-point is formed by a pulverulent mixture of 75 grams of tungsten powder and 25 grams of thorium powder. 90 grams of the pow- 4 der thus obtained are mixed with 10 grams of a mixture of barium, strontium and calcium carbonate. The powder obtained is formed into a paste with the aid of a binder consisting, for ex ample, of a 10% solution of nitro-cellulose and ethylene glycol.

This paste is pressed through a hole so that small cylinders are produced which have a diameter of 2 mms. and a length of from 3 to l mms. These cylinders are provided on a wire of 500 M which acts at the same time as a current supply wire for the gas discharge lamp to be produced.

Example II A dry pulverulent mixture of grams of tungsten powder, 20 grams of thorium powder and 18 grams of a carbonate mixture of barium, strontium and calcium is mixed with 45 crns. of butyl acetate having dissolved in it 1.8% of nitrocellulose. The whole of it is ground in a ball mill for 24 hours.

A cathode core, which may consist, for example, of a straight wire of tungsten on which a helical wire of tungsten is wound, is immersed in this paste, the emitting material being thus applied to this cathode core. After drying this treatment is repeated in the same manner two or three times, whereupon the cathode thus obtained is sintered at a high temperature and is sealed into the lamp.

What we claim is:

1. An electron emitting electrode for an electric discharge device of the type employing an ionizable medium, said electrode comprising a metallic component of the group consisting of tungsten, molybdenum and tantalum, a second metallic component comprising a metal of the group consisting of thorium and hafnium present in a proportion greater than twenty per cent by weight, and a compound of an alkaline earth metal present in a percentage by weight less than that of the metallic components.

2. An electron emitting electrode for an electric discharge device of the type employing an ionizable medium, said electrode comprising at least one metal from the group consisting of tungsten, molybdenum and tantalum, a second metallic component consisting of thorium in a proportion greater than twenty per cent by weight, and a compound of an alkaline earth metal present in a percentage by weightless than that of the respective metallic components.

3. An electron emitting electrode for an electric discharge device of the type employing an ionizable medium, said electrode consisting of a sintered mass comprising by weight sixty-iive per cent powdered tungsten, twenty-five per cent powdered thorium, and ten per cent alkaline earth compound.

GREGORIUS JOHAN LAMBERT. ADRIAAN A. PADMOS.

CIES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

